Ceramics have many excellent properties which make their use as structural materials very attractive. These properties include, among others, high strength, high hardness, wear resistance, high melting temperature, and excellent chemical and thermal stability. However, ceramics are also brittle, a feature which has inhibited their wide use as structural materials. While ceramic materials may be found in selected applications in electronics, air pollution control systems, catalysis and as refractories, their structural use has been limited. New ceramic materials which overcome the brittleness problem would find increased use and application in areas where they have not previously been considered.
The most promising results toward improving ceramic brittleness has been the development of continuous fiber ceramic composites (CFCC) which exhibit high strength and toughness. Examples of successful CFCCs are SiC fiber-reinforced non-oxide, glass, and glass-ceramic composites.
In order to achieve strength and toughness, these non-oxide CFCCs rely upon the presence of, for example, a C or BN film between the reinforcing fiber and the matrix (J. J. Brennan, "Interfacial Characterization of Glass and Glass-Ceramic Matrix/Nicalon SiC Fiber Composite"; Tailoring Multiphase and Composite Ceramics, R. T. Tressler et al., Eds. (Plenum, New York N.Y., 1986), pages 549-560). The role of the interfacial film is to provide weak planes which preferentially debond and slide. The preferential debonding and sliding at the interfaces of CFCCs dissipates energy by internal friction, thus inhibiting crack growth across the interface between the fibers and the ceramic matrix or, in the case of multilayer ceramics, the C or BN interlayer can enhance interface delamination. In the absence of a weak fiber/ceramic matrix interface, the fiber reinforced composites will demonstrate catastrophic fracturing under stress. However, while these non-oxide CFCCs, which contain C, BN or similar materials as an interlayer may exhibit improved toughness, they lack high temperature stability. High temperature oxidation of C, BN and other non-oxide ceramic components degrades the properties of the reinforcing material, and consequently, the properties of the ceramic composite. This limits the application of ceramic compositions containing such materials. An additional drawback to these non-oxide CFCCs is their high fabrication cost.
The search for improved ceramic materials which are not oxidation sensitive has been strongly pursued and is focused on replacing materials such as C and BN with oxide materials such as alumina or other oxides. It has been found that in addition to their oxidation resistance, these oxide materials and the ceramics which contain them have lower thermal conductivity and higher electrical resistivity properties relative to the non-oxide CFCCs. Examples of such oxide containing ceramics may be found in U.S. Pat. Nos. 5,137,852 (the '852 patent) and 5,514,474 (the '474 patent), both to Morgan et al. These patents describe the use of high strength alumina fibers coated with a monazite or xenotime phosphate ('852 patent), or .beta.-alumina or a magnetoplumbite material (.beta.474 patent). The high strength fibers are embedded in the ceramic matrix. The monazite, xenotime, .beta.-alumina or magnetoplumbite coating on the fibers serves as the weak bond interphase material which provides the "planes" of slippage to relieve strain and dissipate energy by internal friction, and thus serves to inhibit crack or fracture growth. In composites containing fibers, the weak interface allows crack deflection and fiber/matrix interface debonding without rupturing the fibers. The strengthened and toughened ceramic resulting from the use of such fibers is thus preserved even though a microcrack has developed in the ceramic material. While the Morgan et al. ceramic materials exhibit improved high temperature oxidation resistivity, and represent an improvement over the C and BN containing ceramics, further improvements in the strength and toughness of ceramic materials are desired and can be achieved by the use of laminates.
Ceramic materials may have either a single matrix composition, into which reinforcing materials may be incorporated, or may be laminated or layered. Laminated ceramic composites with unique and adjustable properties can be achieved by stacking tapes having different ceramic compositions. Reinforcing particulates, fibers and whiskers can also be incorporated into the laminates by adding them to the desired layers. Tough laminated composites can be obtained by introducing ductile layers such as metallic layers, carbon fiber/epoxy layers, or weak C or BN interlayers in between the ceramic layers. For example, Clegg et al., Nature, 347;455-57 (1990) and Acta Metall., 40 11!: 3085-93 (1992), produced a SiC containing laminate; Folsom et al., J Am. Ceramic Soc., 75 11!: 2969-75 (1992) prepared a laminar ceramic/carbon fiber-reinforced epoxy composite; and Baskaran et al., J Am. Ceramic Soc., 76 9!: 2217-24 (1993) and 77 5!: 1249-55 (1994) reported on laminates containing SiC/graphite and SiC/BN. While these laminates have improved toughness, they all contain oxidizable substances and thus have limited high temperature oxidation resistance.
All-oxide composites can solve the problem of high temperature oxidation. From the experience gained in non-oxide CFCCs, fiber-reinforced composites, using oxide fibers, appear to be most promising for all-oxide composites. However, critical issues which need to be solved include producing a creep-resistant oxide fiber, finding a weak oxide interphase which behaves like graphite and boron nitride, and fabricating a dense composite. On the other hand, oxide laminates without the fiber and densification problems are easier fabricate, at a lower cost, for the oxidation-resistant applications. However, until the present invention, the brittleness of oxide laminates limited their structural applications.
It is a purpose of the present invention is to identify high strength, damage tolerant oxide ceramic composites that are formed without the use of oxidation sensitive materials and expensive fibers.
It is also the purpose of the invention to identify oxide containing ceramic laminates which have improved mechanical and thermal properties, and are formed without the use of oxidation sensitive materials or expensive fibers.
It is a further purpose of the present invention to identify high-strength, damage-tolerant oxide ceramic composites or laminates which have improved mechanical properties and which may be further strengthened by the incorporation of particulates, fibers or whiskers into selected layers of the laminate.
It is a purpose of the present invention to describe a method for preparing laminated, high-strength, damage-tolerant ceramic composites of reduced oxidation sensitivity.